Avoid JRuby warning during rehash Rubygems plugin

As it seems, JRuby 1.7 complains on stderr every time you invoke `system`:

    warning: executable? does not in this environment and will return a dummy value

It doesn't seem to complain when backtics are used. It's safe to use
backticks here because `rbenv rehash` doesn't output anything on stdout,
and the exit status of the command is irrelevant.

rbenv.d/exec/gem-rehash/rubygems_plugin.rb

@@ -3,7 +3,7 @@ hook = lambda do |installer|
     # Ignore gems that aren't installed in locations that rbenv searches for binstubs
     if installer.spec.executables.any? &&
         [Gem.default_bindir, Gem.bindir(Gem.user_dir)].include?(installer.bin_dir)
-      system "rbenv", "rehash"
+      `rbenv rehash`
     end
   rescue
     warn "rbenv: error in gem-rehash (#{$!})"